This invention relates to an improved system for edge gated injection molding.
In a typical edge gated system, a number of cavities are located in a cooled cavity plate around a central heated nozzle or bushing. The fact that there is no valve or other shut-off means to assist in melt flow control causes the temperature and heat flow characteristics in the gate area to be of critical importance. The system must be capable of rapidly and repeatably filling the cavities and then opening them to eject the molded product without plugging and without excessive drooling. This requires an insulation or partial insulation between the heated bushing and the cooled cavity plate so that the bushing will remain hot enough to maintain the melt in a molten state and the cavity plate will remain cool enough to quickly solidify the melt when it flows into the cavities.
In the past, this insulation has been provided by providing a space adjacent the valve gate between the bushing and the cavity plate, and allowing it to fill with melt. The melt solidifies, at least adjacent the cool cavity plate, and thus provides a degree of insulation between the bushing and the cavity plate. For instance, the applicant's U.S. Pat. Nos. 3,822,856 entitled "Hot Runner Heater" which issued July 9, 1974 and 4,094,447 entitled "Heater Cast for Multi-Cavity Hot Runner Edge Gate" which issued June 13, 1978 both show systems which carry this one step further with the radial portions of the runner passage being in direct contact with the cavity plate. However, these nozzle seals have previously been used by the applicant in valve gated injection molding systems as disclosed in U.S. Pat. No. 4,043,740 entitled "Injection Molding Nozzle Seal" which issued Aug. 23, 1977 and in U.S. patent application Ser. No. 173,084 filed July 28, 1980, now U.S. Pat. No. 4,286,941, entitled "Injection Molding Nozzle Seal". In both of these applications, the nozzle seal acts to conduct additional heat to the area of the gate which facilitates seating of the valve to improve the reliability of the system and extend the operating life of the valve pin operating mechanism. In fact, in application Ser. No. 173,084, now U.S. Pat. No. 4,286,941, the nozzle seal itself actually forms the gate itself in which the valve pin tip seats.
The fact that these advantages are only available in a valve gated system reduces the possibility that such a seal would be incorporated into other types of molding systems. More particularly, the fact that in an edge gated molding system the cavity plate wall containing the gates is curved points away from the use of these seals in an edge gated system because of the problem of leakage of the pressurized melt. If there is leakage of the melt in the gate area, it will flow into the air gap between the bushing and the cavity plate with the attendant problems mentioned above.